The goal of this research is to determine the neural and peripheral tissue target sites and mechanisms of action of melatonin on body weight and energy metabolism. Melatonin is thought to be the neurochemical transducer of seasonal changes in reproduction, however, melatonin injections, like short day exposure, increase body weight and carcass lipin in Syrian hamsters (Mesocricetus auratus) as well. One set of experiments will examine CNS sites of action for the effects of melatonin on body weight and energy metabolism (including measures of brown adipose tissue thermogenic activity and white adipose tissue lipid deposition). Microknife cuts and radiofrequency lesions in combination with systemic melatonin injections or short day exposure will also be performed. Preliminary data suggest that the paraventricular nucleus may be a primary target site. A second set of experiments will examine peripheral targets of melatonin action such as brown and white adipose tissue and the thyroid gland using surgical denervation of brown fat and chemical throidectomy. A third set of experiments will examine the CNS site(s) and mechanism(s) behind the short day-induced pineal/melatonin independent increases in body weight and energy metabolism using many of the above techniques. Finally, the generality of the target sites for these effects of melatonin on body weight and energy metabolism will be examined in Siberian hamsters (Phodopus sungorus sungorus), a hamster species that shows short day- and melatonin-induced decreases in body weight and carcass lipid. These experiments represent an interdisciplinary approach to the study of naturally-occurring changes in adiposity by examining the interactions of neural and hormonal factors with environmental changes.